The present invention relates to ammonium sulfate nitrate composites useful as fertilizers having desirable levels of nitrate ions, superior stability against detonation, higher density, greater resistance to moisture, and a method for their manufacture.
2. Description of the Related Art
Ammonium sulfate nitrate (ASN), one of the first synthetic fertilizers, has been in continuous use for nearly 100 years providing the important primary and secondary nutrients, nitrogen and sulfur. Nitrogen is provided in part through the nitrate ion, desirable because it is readily adsorbed by many plants and promotes early growth. As historically used, the term xe2x80x9cammonium sulfate nitratexe2x80x9d has not referred to a specific chemical compound with elements in fixed proportions. Rather, it has been used to describe various mixtures of ammonium nitrate and ammonium sulfate. The Association of American Plant Food Officials (AAPFCO), which has assumed the role of monitoring and defining fertilizers, has attempted to bring order to the nomenclature. APPFCO has defined ASN as a double salt of ammonium sulfate and ammonium nitrate in equal molar proportions having a nitrogen content not less than 26%. An equal molar mixture of ammonium sulfate and ammonium nitrate has a nitrogen content of 26.4%.
Despite the AAPFCO definition, the name, ammonium sulfate nitrate, has been used to designate many combinations of ammonium sulfate and ammonium nitrate. See for instance, R. S. Meline, J. Agric. Food Chem., 16(2), 235-240 (1968), where one product has a 30% nitrogen content. U.S. Pat. No. 2,795,495 describes ammonium sulfate nitrate as having an ammonium sulfate/ammonium nitrate mole ratio of 1:2 not 1:1. Great Britain Patent No. 798,690 states that the proportion of ammonium sulfate is not critical and may be used in any proportion necessary to obtain the desired nitrogen level. The use of such terminology has led to confusion between pure double salts and mixtures. Additionally, the order of the words, sulfate and nitrate, are sometimes interchanged in the literature.
A double salt is a distinct compound. The AAPFCO definition implies the existence of a compound consisting of one mole of ammonium sulfate and one mole of ammonium nitrate. A few workers have reported such a compound, however, no 1:1 double salt has been isolated and definitively proven to exist. Nikonova et al., Zhurnal Prikaladnoi Khimii, 15(6), 437-446(1942) critique and correct the works of earlier authors.
The mere mixing of ammonium sulfate and ammonium nitrate does not result in either a reaction or a reaction to completion. Sufficient conditions, including time, must exist to bring about a complete chemical reaction between the two initial salts. However, even under ideal conditions, the reaction of equal molar proportions of ammonium sulfate and ammonium nitrate does not yield an equal molar compound. Instead, the reaction products are double salts of other proportions mixed with unreacted ammonium sulfate and/or ammonium nitrate.
Double salts consisting of NH4SO4xc2x72(NH4NO3) and NH4SO4xc2x73(NH4NO3) (hereinafter the 1:2 double salt and the 1:3 double salt respectively) have been isolated and confirmed. The 1:3 product was isolated from aqueous solution and reported as early as 1909 (Reicher et al., Chemish Weekblad., 3 (Jan.), 51-56 (1909)). Scheinemakers et al. reported in 1910 in the same publication (Volume 6, 1910, pages 51-56) the isolation of a 1:2 double salt as well as the 1:3 double salt from aqueous solutions. The existence of 1:2 and 1:3 double salts have been confirmed by Nikonova (loc. cit.); Itoh, Kogyo Kagaku Zasshi, 63(11), 1913-1916 (1960); Emons et al., Wissenschaftliche. Zeitschrift Techn. Hocksch. Chem. Leuna-Merseburg, 14(3), 295-299 (1972); and Smith et al., J. Agr. Food Chem., 10, 77-78 (1962), among others.
Reported manufacturing processes for ammonium sulfate nitrate describe preparation of uniform fertilizer granules. Most products are simply mixtures of ammonium sulfate and ammonium nitrate rather than specific crystal structures since the reported chemical compositions do not reflect any specific compound. An exception is U.S. Pat. No. 2,762,699 that claims a process for the manufacture of the 1:2 double salt.
Crystallization, granulation and prilling manufacturing methods have been reported. GB 798,690 describes a method of crystallization from an aqueous solution of ammonium sulfate and ammonium nitrate. Granulation processes generally employ temperatures below the melting point of ammonium nitrate (170xc2x0 C.), which assures a product that is incompletely reacted. Examples of granulation processes are described in U.S. Pat. No. 3,635,691, GB 893,389, DE1,039,498, GB 1,259,778, and in the previously mentioned journal article by R. S. Meline, et. al. Prilling processes involve temperatures above the melting point of ammonium nitrate (170xc2x0 C.). Examples are described in Polish Patent No. PL86,766, and in Przem. Chem., 55(12), 611-614 (1976). A small amount of water was added to facilitate the melting of the nitrate. Swedish Patent 70,119 describes a process using up to 10% water with a charge consisting of ammonium sulfate and ammonium nitrate in molar ratio of 0.6:1 and prilling immediately after melting.
Few details on the composition of the products from these processes have been reported. Most of these, if not all, are mixtures consisting of double salts, ammonium sulfate, and ammonium nitrate.
It is clear from many literature references that the products of these processes suffered from low crush strength, adsorption of humidity, and caking. For instance, French Patent No. 1,368,035 stated that ammonium sulfate nitrate fertilizer possessed problems with stability in storage. Belgian Patent No. 388,046 listed several methods that utilized additives to reduce these problems. The susceptibility to caking from absorption of humidity was addressed by adding amphoteric metal oxides. U.S. Pat. No. 2,795,495 described a process to improve the stability of ammonium sulfate nitrate by adding iron salts followed by exposure to ammonia. GB 1,259,778 described a composition incorporating aluminum hydroxide or an aluminum salt to obtain improved anti-caking properties. GB 372,388 improved stability by adding urea and magnesium salts.
J. Turlej, Prz. Chem, 55(12), 611-614 (1976) described efforts that various manufacturers adopted to improve ammonium sulfate nitrate anti-caking stability. BASF, currently the largest producer, reportedly added aluminum, magnesium, and/or calcium compounds. Ruhrchemie added ferrous sulfate.; Victor-Chemische added ferrous sulfate; Uhde Hebernia added ferrous sulfate and phanolite, a mineral. SBA (Belgium) added certain other substances. Turlej""s own work, reported in the same journal, showed the addition of dolomite, aluminum, and/or magnesium compounds to increase stability and eliminate caking.
Several references have reported that ammonium sulfate nitrate always has unreacted starting materials. I. G. Farbenindustries reported in DE. 555,581 and DE 555,902 that free ammonium nitrate is always present. Srinivasa, et al. reported in Technology, 6(1), 21-23 (1969) that the product always contains free ammonium nitrate. Ammonium nitrate is known to be very hygroscopic and it likely contributed to caking problems and particle weakness.
Ammonium nitrate mixed with organic materials such as fuel oil is an important industrial explosive. It has also been linked to terrorist incidents such as the New York World Trade Center and Oklahoma City bombings. It is desirable to render ammonium nitrate less sensitive to detonation. One way to do that is to dilute ammonium nitrate with a relatively inert material. U.S. Pat. No. 3,366,468, 1968, claimed that 5 to 10% of additives such as ammonium phosphates or ammonium sulfate could desensitize and render ammonium nitrate resistant to flame and detonation. However, the pure ammonium sulfate-ammonium nitrate double salts should also be considered as potential explosive materials because the oxygen to nitrogen ratio is favorable for oxidation reactions.
Naoum et al., Zeit. fur das Gesamte Sceiss. Und Sprengstoff., 19, 35-38(1924) reported the heat of explosion (reaction) of ammonium nitrate and mixtures of ammonium nitrate with ammonium sulfate as a function of composition. The latent energies in 64.5 wt. % and 54.8 wt. % mixtures of ammonium nitrate, corresponding to the 1:3 and 1:2 double salts, were 81% and 76% respectively of the explosive energy of pure ammonium nitrate. Data obtained by an independent testing laboratory for Honeywell International show that the 1:3 double salt releases more energy on decomposition than the 1:2 salt. However, Naoun et al. indicate that a completely homogeneous mixture containing less than 40 wt. % ammonium nitrate would probably no longer be explosive. The latent explosive energy was zero at a mole ratio of 1:1 ammonium sulfate/ammonium nitrate (37.7% by weight of ammonium nitrate). Naoum et al., WO 9961395 A1 also showed that the more intimate the mixture of ammonium nitrate and ammonium sulfate, the greater the difficulty of detonation. However, mixtures of ammonium nitrate and large grained ammonium sulfate could produce more gas expansion once detonation was achieved than ammonium nitrate alone. Therefore, an intimate mixture of ammonium nitrate with ammonium sulfate consisting of very small particles is most desirable. Double salts provide intimacy at the molecular level and dispersion at the scale of a few Angstroms. Conditions sufficient for reaction of the ammonium sulfate with the ammonium nitrate maintained for a sufficient period react the ammonium nitrate near completely to the double salts. Such reaction, if carried out with sufficient molar excess of ammonium sulfate, results in a complete or near-complete conversion to the safer 1:2 double salt.
Ammonium sulfate nitrate materials are currently manufactured and sold by BASF and Fertiberia. X-ray diffraction analysis of the BASF material shows a composition consisting on average of 27 wt. % ammonium sulfate, 1 wt % of the 1:2 double salt and 72 wt. % of the 1:3 double salt. The Fertiberia material is 29 wt. % ammonium sulfate, 35 wt. % of the 1:2 double salt and 36 wt. % of the 1:3 double salt. These products appear to be made by a granulation process. The granules are inhomogeneous surface-to-core.
Nutrient value and safety and stability of ammonium sulfate nitrate compositions are competitive properties. Nutrient value increases with increased nitrate content but so also does sensitivity to detonation and sensitivity to moisture. In view of the high penalty for detonation, it is seen that the balance between these properties must be struck where safety is assured. The foregoing considerations lead to the view that what is needed, is an ammonium sulfate nitrate product containing the maximum nitrate content consistent with non-hazardous properties.
The present invention provides a non-explosive ammonium sulfate nitrate composite material comprising by x-ray diffraction analysis about 14 wt. % to about 35 wt. % ammonium sulfate ((NH4)2SO4); about 60 wt. % to about 85 wt. % (NH4)2SO4xc2x72(NH4NO3) double salt; and 0 to about 5 wt. % in combined total (NH4)2SO4xc2x73(NH4NO3) double salt and ammonium nitrate (NH4NO3). The composites are useful as fertilizers, have reduced moisture sensitivity, are not considered hazardous materials under Title 49 of the Code of Federal Regulations, xe2x80x9cTransportationxe2x80x9d, Part 172, xe2x80x9cHazardous Materials Tablexe2x80x9d, Oct. 1, 2000, and are not classified as oxidizers under United Nations Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria, 1995xe2x80x9d, xe2x80x9cSection 34, Classification Procedures, Test Methods and Criteria Relating to Oxidizing Substances of Division 5.1xe2x80x9d.
The invention also provides a method for the production of ammonium sulfate nitrate composite materials comprising the steps of: (a) charging materials comprising ammonium sulfate particles, ammonium nitrate and water to a melting device, wherein the molar ratio of ammonium sulfate to ammonium nitrate is about 0.9:1 to about 1.1:1 and the water is more than 2 wt. % to about 10 wt. % of the charged materials; (b) melting the ammonium nitrate and dissolving at least a portion of the ammonium sulfate particles at a temperature of about 180xc2x0 C. to about 210xc2x0 C.; (c) reacting the charged materials at a temperature of about 180xc2x0 C. to about 210xc2x0 C.; and (d) solidifying the product at a cooling rate of at least about 100xc2x0 C./min.
The invention also includes an ammonium sulfate nitrate composite material prepared by the process comprising the steps of: (a) charging materials comprising ammonium sulfate particles, ammonium nitrate and water to a melting device, wherein the molar ratio of ammonium sulfate to ammonium nitrate is about 0.9:1 to about 1.1:1 and the water is more than 2 wt. % to about 10 wt. % of the charged materials; (b) melting the ammonium nitrate and dissolving at least a portion of the ammonium sulfate particles at a temperature of about 180xc2x0 C. to about 210xc2x0 C.; (c) reacting the charged materials at a temperature of about 180xc2x0 C. to about 210xc2x0 C.; and (d) solidifying the product at a cooling rate of at least about 100xc2x0 C./min.
In the accompanying drawing figures:
FIGS. 1-3 show Cu Kxcex11 x-ray diffraction scans for an ammonium sulfate nitrate composite material comprising by x-ray diffraction analysis, 7.6 wt. % ammonium sulfate, 42.4 wt. % (NH4)2SO4xc2x72(NH4NO3), 45.7 wt. % (NH4)2SO4xc2x73(NH4NO3) and 4.3 wt. % ammonium nitrate.
FIG. 1 shows the x-ray diffraction scan for said ammonium sulfate nitrate composite material in the angular range 18.2xc2x0 to 21xc2x0 2xcex8.
FIG. 2 shows the Cu Kxcex11 x-ray diffraction scan for said ammonium sulfate nitrate composite material in the angular range 30.40xc2x0 to 31.8xc2x0 2xcex8.
FIG. 3 shows the Cu Kxcex11 x-ray diffraction scan for said ammonium sulfate nitrate composite material in the angular range 31.8xc2x0 to 33.8xc2x0 2xcex8.
FIG. 4 is a plot showing the relationship between the water in the charge and the percentage of 1:2 double salt in the product.
FIG. 5 is a plot showing the relationship between the water in the charge and the percentage of 1:3 double salt in the product.
FIG. 6 is a plot showing the relationship between the water in the charge and the percentage of ammonium nitrate in the product.
FIG. 7 is a plot showing the relationship between the water in the product, as-formed, and the percentage of 1:2 double salt in the product.